The CCITT has defined the Asynchronous Transfer Mode (ATM) as the transfer mode for implementing Broadband ISDN (B-ISDN). ATM is based on the use of shod, fixed-length packets called "cells". A cell consists of an information field prefixed with a header for routing and control information.
ATM cells are routed based on the contents of the VPI/VCI fields in the header. The VPI/VCI values are used in intermediate switching nodes in conjunction with routing tables to determine the outgoing link on which the cell should be forwarded. They are only valid for the current link and are in general replaced by a new value at the next section. The routing table in any switching node contains an entry for each VPI/VCI value assigned on each incoming link, with the entry providing a mapping to the appropriate outgoing link and the new VPI/VCI value to be used on that link. The assignment of the VPI/VCI values and construction of the routing table entries are carried out as part of a connection setup procedure. Hence. ATM networks provide basically a connection-oriented virtual circuit service.
Different aspects of ATM and its application are described in the following publications:
a) CCITT Study Group XVIII, Report R34, June 1990. This includes: PA0 b) J. Dupraz et al.: "Principles and Benefits of the Asynchronous Transfer Mode". Electrical Communication, Vol. 64. No. 2/3, 1990, pp. 116-123. PA0 c) J. L. Adams: "The Virtual Path Identifier and Its Application for Routeing and Priority of Connectionless and Connection-Oriented Services". Int. J. of Digital and Analog Cabled Systems, Vol. 1, No. 4 (1988). pp. 257-262.
Draft Recommendation 1.121: Broadband aspects of ISDN. PA1 Draft Recommendation 1.150: B-ISDN ATM functional characteristics. PA1 Draft Recommendation 1.311: B-ISDN general network aspects.
To provide connectionless data services, CCITT defined a Connectionless Service Function (CLSF) grouping which terminates the connectionless protocol and routes cells to a destination user according to routing information included in user cells. However, using this procedure the ATM network merely provides access to the CLSF rather than providing a real connectionless service.
The respective proposal was published also in the above mentioned CCITT report R34, as Draft Recommendation 1.327: B-ISDN Functional Architecture (in particular Annex A "Support of connectionless data services in a B-ISDN").
The routing and switching functions which are performed by the connectionless data services proposed by CCITT only exploit the VPI/VCI switching principles of the ATM to transfer the cells carrying connectionless messages between the users and the CLSF. Within the CLSFs a second layer of switching, called "overlay" switching, is performed based on ST and MID fields in the cell headers.
The disadvantages of this two-level switching concept lay mainly in the use of the CLSF, especially in the case where the two communicating users are attached to the same ATM switch. In this case the cells exchanged between the two users are transmitted twice through the ATM switch. Furthermore, the CLSF has to maintain transient data structures for every message, in order to handle routing and MID fields. Besides, in case of heavy local traffic the CLSF may represent the bottleneck of the network, since all connectionless cells are first directed there before they are distributed.